1. Field of the Invention
The present invention relates to optical projection systems, and in particular to a large-field unit-magnification projection optical system.
2. Description of the Prior Art
Photolithography is presently employed not only in sub-micron resolution integrated circuit (IC) manufacturing, but also to an increasing degree in gold-bumping and other wafer-level IC packaging technologies that require relatively low (i.e., a few microns) resolution, a large depth of focus, and a high throughput. Accordingly, there is an increasing demand for relatively low-resolution in high throughput projection photolithography systems.
The present invention described in the “Detailed Description of the Invention” section below, is related to the optical system described in U.S. Pat. No. 4,391,494 (hereinafter, “the '494 patent”) issued on Jul. 5, 1983 to Ronald S. Hershel and assigned to General Signal Corporation.
FIG. 1 is a cross-sectional diagram of an example optical system 8 according to the '494 patent. The optical system described in the '494 patent and illustrated in FIG. 1 here is a unit-magnification, catadioptric, achromatic and anastigmatic, optical projection system that uses both reflective and refractive elements in a complementary fashion to achieve large field sizes and high numerical apertures. The system is basically symmetrical relative to an aperture stop located at the mirror, thus eliminating odd order aberrations such as coma, distortion and lateral color. All of the spherical surfaces are nearly concentric, with the centers of curvature located close to where the focal plane would be located were the system not folded. Thus, the resultant system is essentially independent of the index of refraction of the air in the lens, making pressure compensation unnecessary.
Optical system 8 includes a concave spherical mirror 10, an aperture stop AS1 located at the mirror, and a composite, achromatic plano-convex doublet lens-prism assembly 12. Mirror 10 and assembly 12 are disposed symmetrically about an optical axis 14. Optical system 8 is essentially symmetrical relative to an aperture stop AS1 located at mirror 10 so that the system is initially corrected for coma, distortion, and lateral color. All of the spherical surfaces in optical system 8 are nearly concentric.
In optical system 8, doublet-prism assembly 12, which includes a meniscus lens 13A, a plano-convex lens 13B and symmetric fold prisms 15A and 15B, in conjunction with mirror 10, corrects the remaining optical aberrations, which include axial color, astigmatism, petzval, and spherical aberration. However, for large-field (i.e., greater than 50 mm×100 mm), broad-spectral-band applications (i.e., for the g, h, i, spectral lines of mercury—436 nm, 405 nm, 365 nm, respectively) and moderately high numerical apertures (e.g., NA≧0.15), optical system 8 cannot produce a sufficiently high-quality image.
Symmetric fold prisms 15A and 15B are used to attain sufficient working space for movement of a reticle 16 and a wafer 18. Optical system 8 includes an object plane OP1 and an image plane IP1, which are separated via folding prisms 15A and 15B. The cost of this gain in working space is the reduction of available field size to about 25% to 35% of the total potential field. In the past, this reduction in field size has not been critical since it has been possible to obtain both acceptable field size and the degree of resolution required for the state-of-the-art circuits.
However, with the increasing demand for larger field sizes, other designs for optical systems capable of supporting larger field sizes and large depth of focus while maintaining resolution and correction over a wide spectral bandwidth are needed.